Auto-Tuned Synchronous Rectifier Controller

ABSTRACT

An apparatus includes a high-pass filter circuit configured to receive a drain-source voltage from a drain node of a synchronous rectifier switch at a secondary-side of a power converter and to generate a filtered drain-source voltage using the received drain-source voltage. A current comparison circuit of the apparatus is configured to receive a current indicative of a current through the synchronous rectifier switch and to generate a current comparison signal using the received current. An auto-tuning controller of the apparatus is configured to turn the synchronous rectifier switch on upon determining a body diode conduction of the synchronous rectifier switch, commence an auto-tuned delay upon determining that the current through the synchronous rectifier switch has changed direction, turn the synchronous rectifier switch off upon expiration of the auto-tuned delay, and update, during a detection window of time, a duration of the auto-tuned delay based on the filtered drain-source voltage.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/811,827, filed Mar. 6, 2020, all of which is hereby incorporated byreference in its entirety for all purposes.

BACKGROUND

Switch-mode power supplies (SMPSs) (“power converters”) are widely usedin consumer, industrial, and medical applications to providewell-regulated power to a load while maintaining high power processingefficiency, tight-output voltage regulation, and reduced conducted andradiated electromagnetic interference (EMI).

Some power converters, such as flyback-converters, include a transformerthat galvanically isolates a primary-side of the power converter from asecondary-side of the power converter. In such power converters, aprimary-side switch of the power converter controls a flow of currentthrough a primary-side winding of the transformer to charge amagnetizing inductance of the transformer. A synchronous rectifierswitch (e.g., a diode or actively controlled switch) on thesecondary-side of the power converter controls a flow of current from asecondary-side winding of the transformer to discharge the energy storedin the magnetizing inductance of the transformer, thereby transferringpower to a load of the power converter.

Some power losses in the primary-side switch relate to a voltage acrossthe primary-side switch and a current through the primary-side switchwhen it is transitioned to an ON-state. Power processing efficiency of apower converter may be increased by minimizing a voltage across theprimary-side switch before the primary-side switch is turned on.

SUMMARY

In some embodiments, an apparatus includes a high-pass filter circuitconfigured to receive a drain-source voltage from a drain node of asynchronous rectifier switch at a secondary-side of a power converterand to generate a filtered drain-source voltage using the receiveddrain-source voltage. A current comparison circuit of the apparatus isconfigured to receive a current indicative of a current through thesynchronous rectifier switch and to generate a current comparison signalusing the received current. An auto-tuning controller of the apparatusis configured to turn the synchronous rectifier switch on upondetermining, using the current comparison signal, a body diodeconduction of the synchronous rectifier switch, commence an auto-tuneddelay upon determining, using the current comparison signal, that thecurrent through the synchronous rectifier switch has changed direction,turn the synchronous rectifier switch off upon expiration of theauto-tuned delay, and update, during a detection window of time, aduration of the auto-tuned delay based on the filtered drain-sourcevoltage.

In some embodiments, a method involves receiving, at high-pass filtercircuit, a drain-source voltage from a drain node of a synchronousrectifier switch at a secondary-side of a power converter. A filtereddrain-source voltage is generated, by the high-pass filter circuit,using the received drain-source voltage. A current indicative of acurrent through the synchronous rectifier switch is received at acurrent comparison circuit. A current comparison signal is generated, bythe current comparison circuit, using the received current. Thesynchronous rectifier switch is turned on upon determining by anauto-tuning controller, using the current comparison signal, that a bodydiode conduction of the synchronous rectifier switch has occurred. Anauto-tuned delay is commenced, by the auto-tuning controller, upondetermining, using the current comparison signal, that the currentthrough the synchronous rectifier switch has changed direction. Thesynchronous rectifier switch is turned off upon expiration of theauto-tuned delay, and a duration of the auto-tuned delay is updated, bythe auto-tuning controller, during a detection window of time, based onthe filtered drain-source voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified circuit schematic of a power converter, inaccordance with some embodiments.

FIG. 2 show simplified plots of signals related to operation of thepower converter shown in FIG. 1, in accordance with some embodiments.

FIG. 3 is a simplified circuit schematic of a synchronous rectifiercontroller for use in the power converter shown in FIG. 1, in accordancewith some embodiments.

FIG. 4 is a portion of an example process for operation of thesynchronous rectifier controller shown in FIG. 3, in accordance withsome embodiments.

FIGS. 5A-B show simplified plots of signals related to operation of thepower converter shown in FIG. 1, in accordance with some embodiments.

FIG. 6 is a portion of the example process of FIG. 4 for operation ofthe synchronous rectifier controller shown in FIG. 3, in accordance withsome embodiments.

FIGS. 7A-B show simplified plots of signals related to operation of thepower converter shown in FIG. 1, in accordance with some embodiments.

DETAILED DESCRIPTION

In accordance with some embodiments, a synchronous rectifier controlleron a secondary-side of a power converter auto-tunes a duration of timethat a negative magnetizing inductance current is developed at aprimary-side switch of the power converter, thereby discharging energystored by a parasitic capacitance of the primary-side switch to reduce adrain-source voltage of the primary-side switch. The primary-side switchis thereafter transitioned to an ON-state having zero or near to zerovoltage developed across the primary-side switch, advantageouslyreducing switching losses of the power converter.

Power converters, such as flyback converters, often include atransformer that galvanically isolates a primary-side of the powerconverter from a secondary-side of the power converter. In such powerconverters, a primary-side switch of the power converter controls a flowof current through a primary-side winding of the transformer to charge amagnetizing inductance of the transformer. A synchronous rectifierswitch on the secondary-side of the power converter controls a flow ofoutput current from a secondary-side winding of the transformer todischarge the energy stored in the magnetizing inductance of thetransformer, thereby transferring power to a load of the powerconverter. In general, the synchronous rectifier switch is in anOFF-state during a time period that the primary-side switch is in anON-state, and the synchronous rectifier switch is generally in anON-state for a portion of the time that the primary-side switch is in anOFF-state.

During a time period that the synchronous rectifier switch is in anON-state, output current from the secondary winding flows to an outputof the power converter. Corresponding to the flow of output current, amagnetizing inductance current of the transformer decreases to zero asenergy stored in the magnetizing inductance is discharged. If thesynchronous rectifier switch remains in an ON-state after themagnetizing inductance current reaches zero, the magnetizing inductancecurrent becomes negative, at which point the magnetizing inductancecurrent will commence discharging a charged parasitic output capacitanceCoss of the primary-side switch. As the output capacitance of theprimary-side switch is discharged, a drain-source voltage of theprimary-side switch is reduced. By controlling how long the negativemagnetizing inductance current flows through the primary winding beforethe synchronous rectifier switch is transitioned to an OFF-state, theprimary-side switch can advantageously attain zero-volt switching (ZVS)or near ZVS. By utilizing ZVS or near ZVS of the primary-side switch,switching losses of the primary-side switch are reduced and powerprocessing efficiency of the power converter is increased as compared toa power converter that does not implement ZVS or near ZVS.

As disclosed herein, the synchronous rectifier controller advantageouslyauto-tunes the duration of time that the synchronous rectifier switchremains in the ON-state after the magnetizing inductance current hastransitioned to a negative current flow to control the discharge amountof the output capacitance Coss of primary-side switch without a prioriinformation regarding an inductance of the transformer, withoutprimary-side measurements of voltage or current, and without receivingcontrol signals from a primary-side controller of the power converter.Because the synchronous rectifier controller is advantageouslycommunicatively isolated from the primary-side of the power converter,design of the power converter is simplified and existing power converterdesigns may make use of the synchronous rectifier controller disclosedherein without requiring changes to be made to the primary sidecontroller.

Additionally, as compared to conventional solutions, some embodimentsdisclosed herein advantageously transition the primary-side switch to anON-state before the drain-source voltage reaches zero volts, therebyachieving near zero-volt switching of the primary-side switch. Byutilizing near ZVS switching, such embodiments advantageously mitigatethe risk of a negative current developing through the primary-sideswitch, thereby reducing the risk of damaging the primary-side switch.

FIG. 1 is a simplified circuit schematic of a flyback power converter(“power converter”) 100, in accordance with some embodiments. Someelements of the power converter 100 have been omitted from FIG. 1 tosimplify the description of the power converter 100 but are understoodto be present. In general, the power converter 100 includes aprimary-side (i.e., an input) configured to receive an input voltageVin′, and a secondary-side (i.e., an output) configured to provide anoutput voltage Vout at the node 124 using the input voltage Vin′. Theprimary-side is coupled to the secondary-side by a transformer 102. Thetransformer 102 transfers power from the primary-side of the powerconverter 100 to the secondary-side of the power converter 100 andgenerally includes a primary winding 104 and a secondary winding 106.The primary-side of the power converter 100 generally includes theprimary winding 104 of the transformer 102, an input voltage filterblock 115, a rectifier block 116 (in the case of AC input), an inputvoltage buffer capacitor C1, a primary-side switch M1 directlyelectrically connected to a node 110 of the primary winding 104, and apower converter controller (“controller”) 118. A magnetizing inductanceL_(M) of the transformer 102 is illustrated as a winding 105. Acompensator 117 is part of a control/feedback path from thesecondary-side of the power converter 100 to the primary-side of thepower converter 100 and is, thus, part of both the primary-side and thesecondary-side. The secondary-side of the power converter 100 generallyincludes the secondary winding 106 of the transformer 102, an outputbuffer circuit 112, a synchronous rectifier switch M2 having a bodydiode, and a synchronous rectifier controller 120. The synchronousrectifier switch M2 is directly electrically connected to the secondarywinding 106 at a node 121. As shown, an output of the power converter100 is configured to be connected to a load R_(L). The feedback paththrough the compensator 117 provides a measurement based on the outputvoltage Vout to the controller 118. Also shown are nodes 107, 111, 122,and 123. Signals related to operation of the power converter 100illustrated in FIG. 1 include a primary-side switch control signalGATE_(M1), a power converter feedback signal FB, the input voltage Vin′,a buffered, filtered, or otherwise conditioned input voltage Vin at thenode 111, a magnetizing inductance current i_(LM), a primary-side switchcurrent i_(M1), a drain-source voltage V_(M1) at a drain node of theprimary-side switch M1 (at the node 110), an output current iout of thepower converter 100, a synchronous rectifier switch control signalGATE_(M2), a synchronous rectifier switch drain-source voltage V_(M2) ata drain node of the synchronous rectifier switch M2, a synchronousrectifier switch current i_(SR) through the synchronous rectifier switchM2, and a received, indicated, or sampled synchronous rectifier switchcurrent i_(M2) that is indicative of the synchronous rectifier switchcurrent i_(SR).

The voltage Vin′ is received at the power converter 100 as analternating current (AC) or direct current (DC) voltage. The inputvoltage filter block 115, the rectifier block 116, and the input buffercapacitor C1 provide the filtered, buffered, rectified, or otherwiseconditioned input voltage Vin to the transformer 102 at the node 111.The primary winding 104 receives the input voltage Vin at the node 111.The primary winding 104 is directly electrically connected in series tothe drain node of the primary-side switch M1, and a source node of theprimary-side switch M1 is electrically coupled to a voltage bias nodesuch as ground. The primary-side switch M1 is controlled at a gate nodeby the primary-side switch control signal GATE_(M1) (e.g., apulse-width-modulation (PWM) signal) generated by the controller 118.The primary-side switch M1 controls, in response to the primary-sideswitch control signal GATE_(M1), the current i_(M1) through the primarywinding 104 to charge the magnetizing inductance L_(M) 105 (asillustrated by the magnetizing inductance current i_(LM)) of thetransformer 102 during a first portion of a switching cycle of the powerconverter 100 (i.e., during an on-time of the primary-side switch M1).The synchronous rectifier switch M2 controls a current flow through thesecondary winding 106 to discharge energy stored by the transformer 102into the output buffer circuit 112 and the load R_(L) during asubsequent portion of the switching cycle (i.e., during an off-time ofthe primary-side switch M1).

To elaborate, when the primary-side switch M1 is enabled by thecontroller 118 during the first portion of the switching cycle, currentflows through the primary winding 104 to the voltage bias node. Thecurrent flow through the primary winding 104 causes energy to be storedin the magnetizing inductance L_(M) 105 and a leakage inductance L_(L)(not shown) of the transformer 102. When the primary-side switch M1 isdisabled in the subsequent portion of the switching cycle, the outputvoltage Vout is generated at the output buffer circuit 112 and isprovided to the load R_(L). The compensator 117 receives the generatedoutput voltage Vout at the node 107 and uses the output voltage Vout togenerate the feedback signal FB which is used to adjust an on-time ofthe primary-side switch M1.

The synchronous rectifier switch M2 provides rectification on thesecondary-side of the power converter 100. When the primary-side switchM1 is in an ON-state, the synchronous rectifier switch M2 is in anOFF-state. After the primary-side switch M1 transitions to an OFF-state,the synchronous rectifier switch M2 transitions to an ON-state. During atime period that the synchronous rectifier switch M2 is in an ON-state,the output current iout flows from the secondary winding 106 to theoutput buffer circuit 112 and to the load R_(L). Corresponding to theflow of output current iout, the synchronous rectifier switch currenti_(SR) flows through the synchronous rectifier switch M2. As the outputcurrent iout flows from the secondary winding 106, the magnetizinginductance current i_(LM) flow decreases to zero. If the synchronousrectifier switch M2 remains in an ON-state after the magnetizinginductance current i_(LM) reaches zero, the magnetizing inductancecurrent i_(LM) becomes negative, at which point the magnetizinginductance current i_(LM) will commence discharging a charged parasiticoutput capacitance Coss of the primary-side switch M1. As the outputcapacitance Coss of the primary-side switch M1 is discharged, thedrain-source voltage V_(M1) of the primary-side switch M1 is reduced.Thus, by controlling how long the negative magnetizing inductancecurrent i_(LM) flows through the primary winding 104 before thesynchronous rectifier switch M2 is transitioned to an OFF-state, theprimary-side switch M1 can be advantageously transitioned to theON-state when the drain-source voltage V_(M1) is at or near zero volts,thereby achieving zero-volt switching (ZVS) or near ZVS of theprimary-side switch M1. By using ZVS or near ZVS of the primary-sideswitch M1, switching losses of the primary-side switch M1 are reducedand a power processing efficiency of the power converter 100 isincreased as compared to a power converter that does not implement ZVSor near ZVS.

As compared to conventional solutions, some embodiments disclosed hereinadvantageously transition the primary-side switch M1 to an ON-statebefore the drain-source voltage V_(M1) reaches zero volts, therebyachieving near zero-volt switching of the primary-side switch M1. Byutilizing near ZVS switching, such embodiments advantageously mitigatethe risk of a negative current developing through the primary-sideswitch M1, which reduces the risk of damaging the primary-side switch M1as compared to conventional solutions. Additionally, as disclosedherein, the synchronous rectifier controller 120 advantageouslyauto-tunes the duration of time that the synchronous rectifier switch M2remains in the ON-state after the magnetizing inductance current i_(LM)has transitioned to a negative current flow to control the dischargeamount of the output capacitance Coss of primary-side switch M1.

The synchronous rectifier controller 120, as shown, is communicativelyisolated from the primary-side of the power converter 100, whichincludes the controller 118 and the primary-side switch M1. Because thesynchronous rectifier controller 120 is communicatively isolated fromthe primary-side of the power converter 100, the synchronous rectifiercontroller 120 does not receive timing signals, control signals,indications of voltage, or indications of current from the primary-sideof the power converter 100. Thus, the synchronous rectifier controller120, as disclosed herein, advantageously does not use primary-sidemeasurements or primary-side control signals to perform auto-tuning ofthe synchronous switch M2.

FIG. 2 show simplified plots 200 of signals related to operation of thepower converter 100 shown in FIG. 1 over a sample period during time t,in accordance with some embodiments. Plot 202 includes a plot of thedrain-source voltage V_(M1) 203 of the primary-side switch M1 over timet, and a first region of interest 204. Plot 205 includes a plot of theprimary-side switch control signal GATE_(M1) 206 and a plot of thesynchronous rectifier switch control signal GATE_(M2) 207 over time t.Plot 208 includes a plot of the primary-side switch current i_(M1) 209and a plot of the magnetizing inductance current i_(LM) 210 over time t.Plot 212 includes a plot of the synchronous rectifier switch currenti_(SR) 213 over time t, and a second region t_(delay) of interest 214.Also shown is, an example duration of an auto-tuned delay t_(delay) 215,and a duration of negative magnetizing inductance current 216.

At the beginning of the sample period shown at the far left of the plots200, the primary-side switch M1 is in an ON-state, as illustrated by anasserted level of the primary-side switch control signal GATE_(M1) 206.Concurrently, the synchronous rectifier switch M2 is in an OFF-state, asillustrated by a de-asserted level of the synchronous rectifier switchcontrol signal GATE_(M2) 207. During the time that the primary-sideswitch M1 is on and the synchronous rectifier switch M2 is off, theprimary-side switch current i_(M1) 209 and the magnetizing inductancecurrent i_(LM) 210 increase as the magnetizing inductance L_(M) 105 ofthe transformer 102 is charged. When the primary-side switch controlsignal GATE_(M1) 206 is de-asserted, the primary-side switch M1transitions to an OFF-state and the primary-side switch current i_(M1)209 quickly falls to zero. Shortly thereafter, body diode conduction ofthe synchronous rectifier switch M2 occurs, as illustrated at the secondregion of interest 214. Upon detecting, by the synchronous rectifiercontroller 120, that body diode conduction of the synchronous rectifierswitch M2 is occurring or has occurred, the synchronous rectifiercontroller 120 transitions the synchronous rectifier switch M2 to anON-state, as shown in the plot 205. Accordingly, the synchronousrectifier switch current i_(SR) 213 rises and the magnetizing inductancecurrent i_(LM) 210 decreases as energy stored in the magnetizinginductance L_(M) 105 of the transformer 102 is discharged into the loadR_(L). During the time period that the synchronous rectifier switch M2remains in an ON-state, the magnetizing inductance current i_(LM) 210continues to decrease. Within the region 216, the magnetizing inductancecurrent i_(LM) 210 and the synchronous rectifier switch current i_(SR)213 both become negative. When the synchronous rectifier switch currenti_(SR) 213 transitions to a negative current (i.e., changes direction),the auto-tuned delay t_(delay) 215 is commenced by the synchronousrectifier controller 120. During the time that the magnetizinginductance current i_(LM) 210 is negative, as illustrated by the region216, charge stored by the parasitic capacitance Coss of the primary-sideswitch M1 is discharged. As illustrated at the first region of interest204, discharging the parasitic capacitance Coss of the primary-sideswitch M1 ultimately reduces the drain-source voltage V_(M1) 203 of theprimary-side switch M1. After an expiration of the auto-tuned delayt_(delay) 215, the synchronous rectifier switch M2 is transitioned to anOFF-state. Thereafter, the primary-side switch M1 is transitioned backto an ON-state. Thus, by advantageously controlling a duration of theauto-tuned delay t_(delay) 215, a corresponding forced duration ofnegative magnetizing inductance current i_(LM) 210 (within the region216) discharges energy stored by the parasitic capacitance Coss of theprimary-side switch M1. Discharging energy stored by the parasiticcapacitance Coss of the primary-side switch M1 reduces the drain-sourcevoltage V_(M1) 203 of the primary-side switch M1 to zero, or near zero(i.e., a value greater than zero volts), before the primary-side switchM1 is transitioned to an ON-state, as illustrated at the first region ofinterest 204.

If the duration of the auto-tuned delay t_(delay) 215 is shorter than anoptimal value, the primary-side switch M1 might be transitioned to anON-state when the drain-source voltage V_(M1) 203 is still substantiallygreater than zero, thereby causing switching losses which decrease apower processing efficiency of the power converter 100. However, if theduration of the auto-tuned delay t_(delay) 215 is too long, a negativecurrent may develop through the primary-side switch which couldpotentially damage the primary-side switch M1. Thus, as disclosedherein, the synchronous rectifier controller 120 advantageouslyauto-tunes the duration of the auto-tuned delay t_(delay) 215 to achievean optimal duration.

FIG. 3 is a simplified circuit schematic providing details of thesynchronous rectifier controller 120 of the power converter 100, inaccordance with some embodiments. Some elements of the synchronousrectifier controller 120 have been omitted from FIG. 3 to simplify thedescription of the synchronous rectifier controller 120 but areunderstood to be present. In general, the synchronous rectifiercontroller 120 includes an auto-tuning controller 302 that generallyincludes delay modules 304 and other modules 306 (e.g., timing andcontrol logic, counter circuits, general processors, programmable logiccircuits, a look-up table, and/or other circuits), a high-pass filtercircuit 308, a current comparison circuit 310, a voltage comparisoncircuit 312, and a gate driver circuit 314. Also shown are the nodes121, 122, and 123 previously described with reference to FIG. 1. In someembodiments, all or a portion of one or more of the components 308, 310,312, and/or 314 are located outside of an integrated circuit thatimplements the synchronous rectifier controller 120.

Signals related to operation of the synchronous rectifier controller 120include the sampled synchronous rectifier switch current i_(M2) at thenode 122, the synchronous rectifier switch drain-source voltage V_(M2)at the node 121, the synchronous rectifier switch control signalGATE_(M2) at the node 123, a current threshold value i_(sw) ^(th) at anon-inverting node of the current comparison circuit 310, a voltagethreshold value V_(sw) ^(th) at an inverting node of the voltagecomparison circuit 312, a high-pass filtered drain-source voltage V_(M2)^(HPF) produced by the high-pass filter circuit 308, a currentcomparison signal C_(currentDetect) produced by the current comparisoncircuit 310, a voltage comparison signal C_(incDelay) produced by thevoltage comparison circuit 312, and a gate control signal C_(Gate)produced by the auto-tuning controller 302.

In some embodiments, the sampled synchronous rectifier switch currenti_(M2), received at an inverting input of the current comparison circuit310, is equal to the synchronous rectifier switch current i_(SR). Inother embodiments, the sampled synchronous rectifier switch currenti_(M2) is proportional to the synchronous rectifier switch currenti_(SR). In yet other embodiments, the sampled synchronous rectifierswitch current i_(M2) is a signal that is indicative of the synchronousrectifier switch current i_(SR) (e.g., a voltage, or a digital signal).The current threshold value i_(sw) ^(th) is an appropriate signal forcomparison against the sampled synchronous rectifier switch currenti_(M2). In some embodiments, the current threshold value i_(sw) ^(th) isequal to, is proportional to, or is representative of zero amps.

The high-pass filter circuit 308 is an analog or digital filter circuitconfigured to receive the synchronous rectifier switch drain-sourcevoltage V_(M2), or a signal indicative of the synchronous rectifierswitch drain-source voltage V_(M2) (e.g., a digital signal, or aproportional signal). The high-pass filter circuit 308 is operable tosubstantially attenuate frequency components of the synchronousrectifier switch drain-source voltage V_(M2) that are less than anon-zero frequency cut-off value (e.g., 5 MHz), and pass frequencycomponents of the synchronous rectifier switch drain-source voltageV_(M2) that are greater than the non-zero frequency cut-off value. Thus,the high-pass filtered drain-source voltage V_(M2) ^(HPF) signal,received at a non-inverting input of the voltage comparison circuit 312,is representative of frequency components (i.e., an instantaneousvoltage component) of the synchronous rectifier switch drain-sourcevoltage V_(M2) that are higher in frequency than the non-zero frequencycut-off value.

In some embodiments, a value for the voltage threshold value V_(sw)^(th) used for comparison against the high-pass filtered drain-sourcevoltage V_(M2) ^(HPF) signal is chosen based on the desired near ZVSvalley voltage as seen on secondary side, i.e., Valley/n, where n is theprimary to secondary transformer turns-ratio. In some embodiments, thevoltage threshold value V_(sw) ^(th) is equal to 2-5 volts, equivalentto valley voltage of 10-30V.

In some embodiments, the gate control signal C_(Gate) produced by theauto-tuning controller 302 is a digital signal configured to control anoutput of the gate driver circuit 314. The gate driver circuit 314level-shifts, buffers, amplifies, or otherwise conditions the gatecontrol signal C_(Gate) to generate the synchronous rectifier switchcontrol signal GATE_(M2).

Operation of the synchronous rectifier controller 120 is described at ahigh level by an example process 400 illustrated in FIG. 4, inaccordance with some embodiments. The particular steps, order of steps,and combination of steps are shown in FIG. 4 for illustrative andexplanatory purposes only. Other embodiments can implement differentparticular steps, orders of steps, and combinations of steps to achievesimilar functions or results. The steps of FIG. 4 are described withreference to the power converter 100 of FIG. 1, relevant signals of theplots 200 of FIG. 2, and details of the synchronous rectifier controller120 shown in FIG. 3.

At step 401, as an initial start condition of the process 400, thesynchronous rectifier switch M2 is in an OFF-state and the primary-sideswitch M1 has transitioned from the ON-state to an OFF-state. At step402, it is determined, e.g., using the current comparison circuit 310,if body diode conduction of the synchronous rectifier switch M2 is orhas been detected. For example, if the synchronous rectifier switchcurrent i_(SR) transitions from a non-zero or zero current level to anegative current level due to body diode conduction of the synchronousrectifier switch M2, as shown at the first region of interest 214 inFIG. 2, the sampled synchronous rectifier switch current i_(M2) willcorrespondingly attain a value that is less than the current thresholdvalue i_(sw) ^(th).

If it is not determined at step 402 that body diode conduction of thesynchronous rectifier switch M2 is or has been detected, flow of theprocess 400 remains at step 402. However, upon determining that thesampled synchronous rectifier switch current i_(M2) is less than thecurrent threshold value i_(sw) ^(th), the current comparison circuit 310generates an asserted current comparison signal C_(currentDetect). Uponreceiving the asserted current comparison signal C_(currentDetect) atthe auto-tuning controller 302 when the synchronous rectifier switch M2is in an OFF-state, it is determined at step 402 that body diodeconduction of the synchronous rectifier switch M2 has occurred and flowcontinues to step 404.

At step 404, the synchronous rectifier switch M2 is transitioned to anON-state by the auto-tuning controller 302 by transmitting an assertedgate control signal C_(Gate) to the gate driver circuit 314. Uponreceiving the asserted gate control signal C_(Gate), the gate drivercircuit 314 transmits a level-shifted, buffered, amplified, or otherwiseconditioned synchronous rectifier gate control signal Gate to a gatenode of the synchronous rectifier switch M2 to transition thesynchronous rectifier switch M2 to the ON-state.

During the time that the synchronous rectifier switch M2 is in theON-state, the primary-side switch M1 remains in the OFF-state, andenergy stored in the magnetizing inductance L_(M) 105 of the transformer102 is discharged (as shown in the plot 208). As the output current ioutflows from the transformer 102 to the output buffer circuit 112 and theload R_(L), a corresponding synchronous rectifier switch current i_(SR)flows through the synchronous rectifier switch M2, as shown in the plot212, and a proportional, representative, or identical sampledsynchronous rectifier switch current i_(M2) is received at an invertingnode of the current comparison circuit 310.

As was shown in the plots 208 and 212, the amplitudes of the magnetizinginductance current i_(LM) and the synchronous rectifier switch currenti_(SR) respectively decrease to zero as the magnetizing inductance L_(M)105 of the transformer 102 is discharged, eventually reaching andcrossing through an amplitude of zero at the beginning of the region216, thereby changing direction. At step 406, it is determined using thecurrent comparison circuit 310 if the synchronous rectifier switchcurrent i_(SR) has changed directions (i.e., is of a negativeamplitude). If it is not determined at step 406 that the synchronousrectifier switch current i_(SR) has changed directions, flow of theprocess 400 remains at step 406. However, if it is determined at step406 that the sampled synchronous rectifier switch current i_(M2) is lessthan the current threshold value i_(sw) ^(th), the current comparisoncircuit 310 generates an asserted current comparison signalC_(currentDetect). Upon receiving the asserted current comparison signalC_(currentDetect) when the synchronous rectifier switch M2 is in anON-state, flow of the process 400 continues to step 408.

At step 408, the auto-tuning controller 302 commences an auto-tuneddelay (e.g., t_(delay) 215 shown in FIG. 2). In some embodiments,commencing the auto-tuned delay t_(delay) involves initializing one ormore timing or delay modules of the delay modules 304 of the auto-tuningcontroller 302. In some embodiments, the auto-tuned delay t_(delay) iscommenced by initializing a delay module of the delay modules 304 tozero and the auto-tuned delay t_(delay) expires when that delay moduleof the delay modules 304 determines that a time equal to the auto-tuneddelay t_(delay) has elapsed. In other embodiments, the auto-tuned delayt_(delay) is commenced by initializing a delay module of the delaymodules 304 to a value corresponding to the auto-tuned delay t_(delay)and the auto-tuned delay t_(delay) expires when that delay moduledetermines that a time equal to the auto-tuned delay t_(delay) haselapsed.

After the auto-tuned delay t_(delay) has commenced, and before theauto-tuned delay t_(delay) has expired, the synchronous rectifier switchM2 remains in an ON-state and a forced negative magnetizing inductancecurrent i_(LM) advantageously discharges energy stored by the parasiticcapacitance Coss of the primary-side switch M1, as shown within theregion representative of the auto-tuned delay t_(delay) 215 of FIG. 2.

At step 410, the auto-tuning controller 302 determines (e.g., using thedelay modules 304) if the auto-tuned delay has expired. If it is notdetermined at step 410 that the auto-tuned delay t_(delay) has expired,flow of the process 400 remains at step 410 and the synchronousrectifier switch M2 remains in an ON-state. If it is determined by theauto-tuning controller 302 at step 410 that the auto-tuned delayt_(delay) has expired, flow continues to step 412. At step 412, thesynchronous rectifier switch M2 is transitioned to an OFF-state by theauto-tuning controller 302 (e.g., by generating a de-asserted C_(Gate)control signal). At step 414, the auto-tuned delay t_(delay) is updated(i.e., “tuned”) by increasing, decreasing, or maintaining a duration ofdelay indicated by the auto-tuned delay t_(delay).

In general, the auto-tuned delay t_(delay) is updated based on a rate ofchange of the drain-source voltage V_(M2) of the synchronous rectifierswitch M2 after the synchronous rectifier switch M2 is transitioned toan OFF-state. As previously described, an instantaneous voltagecomponent of drain-source voltage V_(M2) is indicated by the high-passfiltered drain-source voltage V_(M2) ^(HPF). If the instantaneousvoltage component of drain-source voltage V_(M2) quickly increases to apeak value after the synchronous rectifier switch M2 is transitioned toan OFF-state, the duration of delay indicated by the auto-tuned delayt_(delay) is increased in order to increase the negative magnetizinginductance current i_(LM), thereby increasing the amount of energydischarged from the parasitic capacitance Coss of the primary-sideswitch M1 during the next switching cycle. If, instead, theinstantaneous voltage component of the drain-source voltage V_(M2)slowly increases to a peak value after the synchronous rectifier switchM2 is transitioned to an OFF-state, the duration of delay indicated bythe auto-tuned delay t_(delay) is decreased. By decreasing the durationof delay indicated by the auto-tuned delay t_(delay), the duration ofnegative magnetizing inductance current i_(LM) is correspondinglydecreased, thereby decreasing the amount of energy discharged from theparasitic capacitance Coss of the primary-side switch M1 such that amaximum discharge amount of the parasitic capacitance Coss is less thana full discharge amount (e.g., to prevent a potentially damagingnegative current through the primary-side switch M1 from occurring).Thus, the synchronous rectifier controller 120 advantageously determinesan optimal duration of forced magnetizing inductance current i_(LM) toachieve ZVS or near-ZVS of the primary-side switch M1 without receivingprimary-side measurements or control signals of the power converter 100and without requiring a priori indications of an inductance of thetransformer 102. As such, an existing power converter design can beeasily and affordably modified to include the synchronous rectifiercontroller 120.

Additional details regarding step 414 are described with respect toFIGS. 5A-6. FIG. 5A shows simplified plots 500 of signals related tooperation of the power converter 100 shown in FIG. 1 and details of thesynchronous rectifier controller 120 shown in FIG. 3 over time t, inaccordance with some embodiments. Plot 500 includes a plot of thevoltage comparison signal C_(incDelay) 502, a plot of the synchronousrectifier switch current i_(SR) 504, a plot of the voltage thresholdvalue V_(sw) ^(th) 506, a plot of the high-pass filtered drain-sourcevoltage V_(M2) ^(HPF) 508, a plot of the synchronous rectifier switchdrain-source voltage V_(M2) 510, and a plot of the synchronous rectifierswitch M2 gate control signal C_(Gate) 512 over time t. Also shown is aplot of an example duration of the auto-tuned delay t_(delay) 514 and aplot of an example duration of a detection window t_(detect) 516. Theexample shown in the plots 500 generally illustrates a portion of aswitching cycle of the synchronous rectifier switch M2, occurring atstep 414 of the process 400, in which the instantaneous voltagecomponent (i.e., V_(M2) ^(HPF) 508) of the drain-source voltage V_(M2)510 of the synchronous rectifier switch M2 quickly increases to a peakvalue after the synchronous rectifier switch M2 is transitioned to anOFF-state, thereby exceeding the voltage threshold value V_(sw) ^(th)506 before an expiration of the detection window t_(detect) 516.Accordingly, the duration of delay indicated by the auto-tuned delayt_(delay) is increased, as triggered by an asserted level of the voltagecomparison signal C_(incDelay) 502, in order to increase a duration offorced negative magnetizing inductance current i_(LM).

FIG. 5B shows simplified plots 520 of signals related to operation ofthe power converter 100 shown in FIG. 1 and details of the synchronousrectifier controller 120 shown in FIG. 3 over time t, in accordance withsome embodiments. Plot 520 includes a plot of the voltage comparisonsignal C_(incDelay) 522, a plot of the synchronous rectifier switchcurrent i_(SR) 524, a plot of the voltage threshold value V_(sw) ^(th)526, a plot of the high-pass filtered drain-source voltage V_(M2) ^(HPF)528, a plot of the synchronous rectifier switch drain-source voltageV_(M2) 530, and a plot of the gate control signal C_(Gate) 532 over timet. Also shown is a plot of an example duration of the auto-tuned delayt_(delay) 534 and a plot of an example duration of the detection windowt_(detect) 536. The example shown in the plots 520 generally illustratesa portion of the switching cycle of the synchronous rectifier switch M2,occurring at step 414 of the process 400, in which the instantaneousvoltage component (i.e., V_(M2) ^(HPF) 528) of the drain-source voltageV_(M2) 530 does not quickly increase to a peak value after thesynchronous rectifier switch M2 is transitioned to an OFF-state, andtherefore does not exceed the voltage threshold value V_(sw) ^(th) 526before an expiration of the detection window t_(detect) 536.Accordingly, the duration of delay indicated by the auto-tuned delayt_(delay) 534 is decreased, as triggered by a de-asserted level of thevoltage comparison signal C_(incDelay) 522, in order to decrease aduration of forced negative magnetizing inductance current i_(LM).

Details of step 414 of the process 400 are illustrated in FIG. 6. Theparticular steps, order of steps, and combination of steps are shown inFIG. 6 for illustrative and explanatory purposes only. Other embodimentscan implement different particular steps, orders of steps, andcombinations of steps to achieve similar functions or results. The stepsof FIG. 6 are described with respect to the power converter 100 of FIG.1, details of the synchronous rectifier controller 120 shown in FIG. 3,and relevant signals of the plots 500 and 520 of FIGS. 5A-B.

At step 602, as an initial start condition of step 414, the primary-sideswitch M1 is in an OFF-state and the synchronous rectifier switch M2 hastransitioned from the ON-state to an OFF-state (i.e., at step 412 ofFIG. 4). At step 604, a detection window of time (e.g., t_(detect)516/536) is commenced. In some embodiments, commencing the detectionwindow t_(detect) involves initializing one or more timing or delaymodules of the delay modules 304 of the auto-tuning controller 302. Insome embodiments, the detection window t_(detect) is commenced byinitializing a delay module of the delay modules 304 to zero and thedetection window t_(detect) expires when that delay module determinesthat a time equal to the detection window t_(detect) has elapsed. Inother embodiments, the detection window t_(detect) is commenced byinitializing a delay module of the delay modules 304 to a valuecorresponding to the detection window t_(detect) and the detectionwindow t_(detect) expires when that delay module determines that a timeequal to the detection window t_(detect) has elapsed. In general, thedetection window (e.g., t_(detect) 516/536) is set to a value based on amaximum quasi-resonant half-period. In some embodiments, the detectionwindow t_(detect) is equal to 100 ns (e.g., for a >=300 Mhz switchingfrequency) or 1000 us (e.g., for a <300 kHz switching frequency).

At step 606, it is determined if the detection window t_(detect) hasexpired. If it is determined at step 606 that the detection windowt_(detect) has not yet expired, flow continues to step 608. At step 608,it is determined if the high-pass filtered drain-source voltage V_(M2)^(HPF) of the synchronous rectifier switch drain-source voltage V_(M2)(i.e., an instantaneous voltage component) is greater than the voltagethreshold value V_(sw) ^(th). If it is determined at step 608 that thehigh-pass filtered drain-source voltage V_(M2) ^(HPF) of the synchronousrectifier switch drain-source voltage V_(M2) is greater than the voltagethreshold value V_(sw) ^(th) (e.g., as shown in plot 500 where thehigh-pass filtered drain-source voltage V_(M2) ^(HPF) 508 crosses abovethe voltage threshold value V_(sw) ^(th) 506), an asserted voltagecomparison signal C_(incDelay) 502 is generated by the voltagecomparison circuit 312, as shown in the plots 500, and flow continues tostep 610. At step 610, the delay indicated by the auto-tuned delayt_(delay) is increased by the auto-tuning controller 302 in response toreceiving the asserted voltage comparison signal C_(incDelay) 502. If itis not determined at step 608 that the high-pass filtered drain-sourcevoltage V_(M2) ^(HPF) of the synchronous rectifier switch drain-sourcevoltage V_(M2) is greater than the voltage threshold value V_(sw) ^(th),flow returns to step 606. At step 606, if it is determined that thedetection window t_(detect) has expired (i.e., without having determinedthat the high-pass filtered drain-source voltage V_(M2) ^(HPF) of thesynchronous rectifier switch drain-source voltage V_(M2) is greater thanthe voltage threshold value V_(sw) ^(th)), the delay indicated by theauto-tuned delay t_(delay) is decreased by the auto-tuning controller302. Flow from either of steps 610 or 614 proceed to step 616, whichconcludes the illustrated portion of step 414 of the process 400.

FIG. 7A shows experimental results 700 of signals related to operationof a power converter that is similar to the power converter 100 shown inFIG. 1 using an auto-tuning method similar to, or the same as, theprocess 400, in accordance with some embodiments. The experimentalresults 700 include a plot 702 of the primary-side switch drain-sourcevoltage V_(M1) 703 over time t, a dashed line 704 indicative of aminimum switching voltage of the primary-side switch M1 over time t, anda first region of interest 705. A plot 706 illustrates a plot of thesynchronous rectifier switch control signal GATE_(M2) 707, and a plot ofthe primary-side switch control signal GATE_(M1) 708 over time t. A plot709 includes a plot of the magnetizing inductance current i_(LM) 710 anda plot of the primary-side switch current i_(M1) 711 over time t. FIG.7B continues the experimental results 700 of FIG. 7A, in accordance withsome embodiments. A plot 722 includes a plot of the synchronousrectifier switch current i_(SR) 723 over time t. A plot 724 includes aplot of the synchronous rectifier switch drain-source voltage V_(M2) 725over time t, a dashed line 726 indicative of a primary side switchvalley voltage, and a second region of interest 727 corresponding to thefirst region of interest 705 of the plot 702. The closer the dashed line726 is to a peak voltage of the synchronous rectifier switchdrain-source voltage V_(M2) 725, the closer the power converter 100 isto zero-voltage switching of the primary-side switch M1. As shown by theregion of interest 705 of FIG. 7A, auto-tuning a duration of forcednegative magnetizing current i_(LM), by the synchronous rectifiercontroller 120, zero voltage switching, or near zero voltage switchingof the primary-side switch M1 is achieved.

Reference has been made in detail to embodiments of the disclosedinvention, one or more examples of which have been illustrated in theaccompanying figures. Each example has been provided by way ofexplanation of the present technology, not as a limitation of thepresent technology. In fact, while the specification has been describedin detail with respect to specific embodiments of the invention, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing, may readily conceive of alterations to,variations of, and equivalents to these embodiments. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers all such modificationsand variations within the scope of the appended claims and theirequivalents. These and other modifications and variations to the presentinvention may be practiced by those of ordinary skill in the art,without departing from the scope of the present invention, which is moreparticularly set forth in the appended claims. Furthermore, those ofordinary skill in the art will appreciate that the foregoing descriptionis by way of example only, and is not intended to limit the invention.

What is claimed is:
 1. An apparatus comprising: a high-pass filtercircuit configured to receive a drain-source voltage from a drain nodeof a synchronous rectifier switch at a secondary-side of a powerconverter and to generate a filtered drain-source voltage using thereceived drain-source voltage; and an auto-tuning controller configuredto: turn the synchronous rectifier switch on upon detecting a body diodeconduction of the synchronous rectifier switch; commence an auto-tuneddelay upon determining that a current through the synchronous rectifierswitch has changed direction; turn the synchronous rectifier switch offupon expiration of the auto-tuned delay; and update, during a detectionwindow of time, a duration of the auto-tuned delay based on the filtereddrain-source voltage.
 2. The apparatus of claim 1, wherein: a voltage ata drain node of a primary-side switch of the power converter is reduced,when the synchronous rectifier switch is on, by an amount correspondingto the duration of the auto-tuned delay.
 3. The apparatus of claim 2,wherein: a maximum duration of the auto-tuned delay corresponds to adrain-source voltage at the drain node of the primary-side switch thatis greater than zero volts.
 4. The apparatus of claim 2, wherein: theauto-tuning controller is communicatively isolated from a signalrepresentative of the voltage at the drain node of the primary-sideswitch.
 5. The apparatus of claim 1, wherein the auto-tuning controlleris further configured to: update the duration of the auto-tuned delay byincreasing the duration of the auto-tuned delay upon determining thatthe filtered drain-source voltage surpassed a voltage threshold valueduring the detection window of time; and update the duration of theauto-tuned delay by decreasing the duration of the auto-tuned delay upondetermining that the filtered drain-source voltage did not surpass thevoltage threshold value during the detection window of time.
 6. Theapparatus of claim 5, further comprising: a voltage comparison circuitconfigured to generate a voltage comparison signal based on a comparisonof the filtered drain-source voltage to the voltage threshold value;wherein the auto-tuning controller is further configured to: determine,upon receiving a first level of the voltage comparison signal, that thefiltered drain-source voltage surpassed the voltage threshold value; anddetermine, upon receiving a second level of the voltage comparisonsignal, that the filtered drain-source voltage did not surpass thevoltage threshold value.
 7. The apparatus of claim 1, furthercomprising: a current comparison circuit configured to: generate acurrent comparison signal by comparing a current indicative of a currentthrough the synchronous rectifier switch to a current threshold; whereinthe auto-tuning controller is further configured to: determine that thebody diode conduction of the synchronous rectifier switch has occurredupon receiving a first level of the current comparison signal when thesynchronous rectifier switch is in an OFF-state; and determine that thecurrent through the synchronous rectifier switch has changed directionupon receiving the first level of the current comparison signal when thesynchronous rectifier switch is in an ON-state.
 8. The apparatus ofclaim 7, wherein: the current threshold corresponds to a current ofabout zero amps.
 9. The apparatus of claim 1, wherein: the powerconverter comprises a transformer having a primary-side winding on aprimary side of the power converter and a secondary-side winding on thesecondary-side of the power converter; and the synchronous rectifierswitch is configured to be electrically coupled to the secondary-sidewinding.
 10. The apparatus of claim 9, wherein: a primary-side switch iselectrically coupled to the primary-side winding; and a duration of aforced negative magnetizing inductance current at a drain node of theprimary-side switch corresponds to the duration of the auto-tuned delay.11. The apparatus of claim 10, wherein: the auto-tuning controller iscommunicatively isolated from each of the drain node, a source node, anda gate node of the primary-side switch.
 12. The apparatus of claim 10,wherein: an increase in the duration of the auto-tuned delay correspondsto an increased discharge amount of a parasitic capacitance of theprimary-side switch; and a decrease in the duration of the auto-tuneddelay corresponds to a decreased discharge amount of the parasiticcapacitance of the primary-side switch.
 13. The apparatus of claim 12,wherein: a maximum duration of the auto-tuned delay corresponds tomaximum discharge amount of the parasitic capacitance of theprimary-side switch; and the maximum discharge amount of the parasiticcapacitance of the primary-side switch is less than a full dischargeamount of the parasitic capacitance of the primary-side switch.
 14. Amethod comprising: receiving, at high-pass filter circuit, adrain-source voltage from a drain node of a synchronous rectifier switchat a secondary-side of a power converter; generating, by the high-passfilter circuit, a filtered drain-source voltage using the receiveddrain-source voltage; turning the synchronous rectifier switch on upondetermining by an auto-tuning controller that a body diode conduction ofthe synchronous rectifier switch has occurred; commencing, by theauto-tuning controller, an auto-tuned delay upon determining that acurrent through the synchronous rectifier switch has changed direction;turning the synchronous rectifier switch off upon expiration of theauto-tuned delay; and updating, by the auto-tuning controller, during adetection window of time, a duration of the auto-tuned delay based onthe filtered drain-source voltage.
 15. The method of claim 14, wherein:a voltage at a drain node of a primary-side switch of the powerconverter is reduced, when the synchronous rectifier switch is on, by anamount corresponding to the duration of the auto-tuned delay.
 16. Themethod of claim 15, wherein: a maximum duration of the auto-tuned delaycorresponds to a drain-source voltage at the drain node of theprimary-side switch that is greater than zero volts.
 17. The method ofclaim 15, wherein: the auto-tuning controller is communicativelyisolated from a signal representative of the voltage at the drain nodeof the primary-side switch.
 18. The method of claim 14, furthercomprising: updating the duration of the auto-tuned delay by increasingthe duration of the auto-tuned delay upon determining that the filtereddrain-source voltage surpassed a voltage threshold value during thedetection window of time; and updating the duration of the auto-tuneddelay by decreasing the duration of the auto-tuned delay upondetermining that the filtered drain-source voltage did not surpass thevoltage threshold value during the detection window of time.
 19. Themethod of claim 14, wherein: the duration of the auto-tuned delaycorresponds to a duration of a forced negative magnetizing inductancecurrent at a drain node of a primary-side switch of the power converter.20. The method of claim 19, wherein: the auto-tuning controller iscommunicatively isolated from each of the drain node, a source node, anda gate node of the primary-side switch.